realistically, it would depend if the armor was made specifically for stopping bullets and how good the guns was
I don't understand your meaning. That's how it works now. Naturally, anti-bullet armor should recieve bonuses against guns and crappy guns should recieve a fitting penalty to their stats. Besides, how is the magical bonus increase supposed to symbolize how good the armor is against bullets? Granted, magical armor does better, but in your example there were completely mundane armor and shields. They didn't have enchantments on them, they simply either were gunproof or the guns used against them were subpar. Enchantments and modifications on an item are separate. You could have a magically enchanted(+1, if you prefer), mundanely gunproof shield, for instance.
guns should be done on a fallout 1,2 and NV note and have them have to over come AC with every additional +1 counting as 5 AC and every five levels of +1 increasing by 2 like so.
Overcoming AC in this case doesn't make sense under D&D rules. Think about it. A tower shield gives the most AC out of all shields, and it's made of wood. Small shields and Heavy shields give the same AC bonus each no matter what they're made of. AC bonus from shields and armor are based on how well it protects your body from attacks it CAN wade off, not the material. It's not a stat of the equipment, it's a bonus you get from it. When it comes to penetrating armor(that is, something you do to the armor itself, not the wearer), it's bonus to AC doesn't matter in the slightest.
A Tower Shield is big, hence it defends you more than a buckler, even though bucklers are made of steel. A proper gun would still penetrate it like a sheet of paper because it's still just
wood, not some kind of special, famed tower shield wood which somehow gives more AC than the other kinds of wood.